A prior art accelerometer with high performance potential is described in U.S. Pat. No. 3,702,073. The accelerometer comprises three primary components, a reed, and upper and lower stators or magnetic circuits between which the reed is supported. The reed includes a movable paddle that is suspended via flexures to an outer annular support ring, such that the paddle can pivot with respect to the support ring. The paddle, flexures and support ring are commonly provided as a unitary structure composed of fused quartz. A plurality of mounting pads are formed at spaced-apart positions around the upper and lower surfaces of the support ring. These mounting pads mate with inwardly facing surfaces of the upper and lower stators when the accelerometer is assembled.
Both upper and lower surfaces of the paddle include capacitor plates and force rebalance coils, also known as torque coils. Each force rebalance coil is positioned such that its central axis is normal to the paddle, and parallel to the sensing axis of the accelerometer. Each stator is generally cylindrical, and has a bore provided in its inwardly facing surface. Contained within the bore is a permanent magnet. The bore and permanent magnet are configured such that an associated one of the force rebalance coils mounted on the paddle fits within the bore, with the permanent magnet being positioned within the cylindrical core of the coil. Current flowing through the coil produces a magnetic field that interacts with the permanent magnet, to produce a force on the paddle. Also provided on the inwardly facing surfaces of the stators are capacitor plates configured to form capacitors with the capacitor plates on the top and bottom surfaces of the paddle. Movement of the paddle with respect to the upper and lower stators results in a differential capacitance change.
In operation, the accelerometer is affixed to an object whose acceleration is to be measured. Acceleration of the object along the sensing axis results in pendulous, rotational displacement of the paddle, coils and capacitor plates (collectively referred to as the "proof mass") with respect to the support ring and the stators. The resulting differential capacitance change caused by this displacement is sensed by a feedback circuit. In response, the feedback circuit produces a current that, when applied to the force rebalance coils, tends to return the proof mass to its neutral position. The magnitude of the current required to maintain the proof mass in its neutral position provides a measure of the acceleration along the sensing axis.
An important characteristic of an accelerometer of the type described above is its immunity to errors due to thermal stress. Thermal stress results from the fact that different parts of the accelerometer are composed of materials that have different coefficients of thermal expansion. For example, the reed is preferably composed of fused quartz, whereas the coil is typically composed of copper, and may be mounted on an aluminum coil form. The coefficient of thermal expansion of fused quartz is 0.5 ppm/.degree.C., while the coefficients of expansion of copper and aluminum are 17 and 23 ppm/.degree.C., respectively. Thus temperature change will result in stress at the paddle/coil, or paddle/coil form interface. This stress can warp the paddle and the flexures that mount the paddle, and result in offset and hysteresis errors in the accelerometer output.
One traditional approach to minimize temperature induced stress at the coil/paddle interface is to mount the coil or coil form on a support that is attached to the paddle only at a small circular area near the central axis of the coil. This approach minimizes the difference between the coil and paddle movement caused by a given temperature change. Another prior approach to coil mounting that reduces stress is shown in U.S. Pat. No. 4,697,455. In the accelerometer depicted therein, a plurality of mounting fingers descend from the coil form, and attach to a base that in turn is mounted to the paddle. Although this design effectively removes stress due to thermal expansion mismatch, it is a relatively heavy design that adds a significant amount of mass to the proof mass. There is therefore a need for a coil mounting approach that minimizes thermal stress, but that is also light in weight and thereby well adapted for use in high g accelerometers.